Portable electronic devices such as cellular phones and smartphones recently have grown in demand. Electronic devices as above are equipped with small, slim imaging units.
The imaging units each generally include a solid-state imager, and optical elements such as lenses. The solid-state imager is exemplified by CCD image sensors and CMOS image sensors.
For more efficient production, optical elements to be on the electronic devices require such heat resistance and thermal yellowing resistance as to allow the optical elements to be mounted on the devices by reflow soldering. In addition, the optical elements require still higher levels of heat resistance (about 270° C.) and thermal yellowing resistance, because the use of lead has recently been restricted with consideration for the environment, and soldering using lead-free solder becomes practical.
The optical elements such as lenses are preferably derived from cationically curable compositions as materials. This is because the cationically curable compositions resist cure inhibition by oxygen and less shrink upon curing, as compared with radically curable compositions.
The cationically curable compositions each include a cationically curable compound and a cationic-polymerization initiator. Known examples of the cationic-polymerization initiator include antimony, phosphorus, and borate initiators (Patent Literature (PTL) 1 to 4). Among them, antimony initiators have been widely used. This is because the antimony initiators have excellent cationic polymerization activities and can form cured products even as thin films having a thickness of 50 μm or less (namely, offer excellent curability to form a thin film (thin-film curability). In addition, the resulting cure products can maintain their shapes (namely, have excellent heat resistance) and resist yellowing (namely, have excellent thermal yellowing resistance) even under high-temperature conditions such as reflow soldering conditions.
Disadvantageously, however, the antimony initiators are designated as deleterious substances and have safety problems.
Known phosphorus initiators include onium salts containing a PF6− anion. These initiators offer excellent thin-film curability, but disadvantageously give cured products that are susceptible to yellowing under high-temperature conditions. The special phosphorus initiators disadvantageously offer inferior thin-film curability.
Known borate initiators include onium salts containing a B(C6F5)4− or BF4− anion. Among them, the onium salts containing a BF4− anion disadvantageously have low cationic polymerization activities and offer inferior thin-film curability. In contrast, the onium salts containing a B(C6F5)4− anion offer excellent thin-film curability, but disadvantageously give cured products that are susceptible to yellowing under high-temperature conditions.